Not sure whether this is what mamaKristin had in mind, but one reason to pre-cook meat is for browning - since you're usually cooking it in liquid in the slow cooker, the meat is essentially boiling/poaching/steaming, and so won't get that nice brown sear. (insert a bunch of scientific stuff about the Maillard reaction here)
When I make beef stew, I dredge my chunks of beef in flour, then briefly saute them so they have a brown "crust" before throwing them into the slow cooker with the veggies, broth, etc. I think it gives a better flavor and nicer appearance.
Oh - another reason: to drain off fat! I made chili last week, and didn't want too much liquidy fat in it. So I pre-browned my ground meat in a skillet, then drained off and discarded the fat before putting the meat and chili stuff into the slow cooker.
Hope this is helpful!
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Mommy of Dread Pirate Leonidas, 4-14-13
Sir Landis of Lines, 9-9-11
and Bean, gone Aug 2010 at 18 weeks